The goals of the proposed work are a) the purification of human and bovine milk-derived growth factors, b) the preparation of antibodies of milk-derived factors, and c) the use of milk as a replacement for serum for the growth of cells in culture. The growth factor activity of both bovine and human milk is dependent on the point in the lactation period at which the milk is collected. This temporal dependence is particularly striking in bovine milk. The growth factor activity is highest in colostrum obtained within 24 hours after birth of a calf. Samples of milk obtained 32 hours and 60 hours after birth are 20% and 1% as active respectively as is a sample obtained 8 hours after birth in stimulating DNA synthesis. No activity is detectable 3 days after birth or thereafter. In human milk growth factor activity declines by about 80% in 3 weeks but never totally disappears. A scheme has been devised for the potential purification of both bovine colostrum and human milk. The steps in consecutive order are as follows: acid precipitation, DEAE sephadex chromatography, and preparative electrophoresis, isoelectric focusing, sephadex gel filtration, and high pressure liquid chromatography. Milk is also being used as a substitute for serum in the growth of cells in culture. Milk alone is sufficient for the growth of transformed cells but plates must be precoated with fibronectin for the growth of normal cells in milk. The proposed studies plan to use growth in milk as a technique for determining nutritional requirements for various cells in culture.